This invention relates in general to earth boring equipment and in particular to a stabilizer for centering a drill string in large diameter shafts.
One method of boring large diameter shafts, approximately 4 to 15 feet in diameter, includes rotating a large drill bit by a string of drill pipe. The drill pipe is normally much smaller in diameter than the shaft or the drill bit. Downward thrust is applied to the bit by large weights that surround the drill string and rest on the upper side of the drill bit. Because of some flexibility in the drill pipe, and the clearances between the weight stack and shaft wall, the bits have a tendency to deviate from a straight path.
To restrict deviation and keep the bit on course, a stabilizer is placed in the weight stack normally near the top. One type of stabilizer known to applicant has a cylindrical frame or hub that surrounds the drill pipe and is sandwiched between two weights. Extending from the central hub are radial arms with vertically oriented rollers mounted at the outer ends for rolling contact with the wall of the shaft. This type of stabilizer uses antifriction bearings in the rollers, which require extensive structure for lubrication and protection from the drilling fluid. Also, the weight of the rollers necessitates heavy supporting structure and makes them difficult to change. Furthermore, intermittent contact between the rapidly moving rollers and the shaft wall can promote a sloughing of the shaft wall.
A non-rotating stabilizer is shown in U.S. patent application Ser. No. 12,277, filed Feb. 15, 1979, the inventors being John M. Mullins and Joseph L. Kelly, Jr., now U.S. Pat. No. 4,293,039, issued Oct. 6, 1981. In that application, the centralizer structure does not rotate with the drill string. It has pads that frictionally engage the walls of the shaft when the bit wanders off-center. The pads are carried by an outer hub that encircles an inner hub. The inner hub is carried rigidly by the drill string for rotation therewith. A bearing between the inner and outer hubs allows the outer hub and centralizer arms to remain in non-rotating position as the drill pipe rotates. The bearing constitutes a number of resilient pads that frictionally engage upper and lower frusto-conical rings. The pads are lubricated by fluid in the shaft.
While the device of the above-mentioned patent application is successful, it would be desirable to reduce the sliding friction between the pads and frusto-conical rings so as to reduce consumption of the available drilling torque.